Telecommunication network operators are regularly tasked with evaluating performance of user equipment (UE) devices, particularly UE devices newly introduced for use in telecommunication applications operating over the operators' networks. Typically, UE devices are assembled by manufacturing partners of the operators and delivered for evaluation. Different specifications must be met for different subcomponents of UE devices. For example, displays and cameras for UE devices such as cell phones and tablets must function across a broad range of lighting conditions. For cameras, various tests are required, including measurement of camera response to different levels of brightness in lighting conditions.
Most cameras for UE devices include an automatic exposure control feature. Automatic exposure control is convenient for the end user, because it allows use of the camera in lighting conditions ranging from the high intensity of broad daylight to the low intensity of dimly lit rooms. Automatic exposure control adjusts gain, exposure time, and/or aperture based on the lighting conditions so that ideally the overall picture has the objectively correct brightness. Adjustment takes a certain response time, typically on the order of a fraction of a second. The region in the image that is used to set the exposure varies from the full image, a matrix of dots, a small spot at a fixed location or a dynamically determined area (for example where a face has been detected).
Unfortunately, automatic exposure control hinders the measurement of the response curves generated when testing cameras for inclusion in UE devices. This is because, when the camera captures a test image of uniform brightness, the automatic exposure control of the camera will adjust the exposure to make the image appear grayish. What is needed are improved methods and systems for measuring brightness response of a camera operating in automatic exposure mode.